This invention relates to screening machines of the type used to separate or classify mixtures of particles of different sizes. More particularly, the invention relates to an improved screen panel for use within such a screening machine.
In screening machines of the type described, a screen (which may be woven, an aperture plate or another design) is mounted in what is often called a “screen frame” or “screen deck” which includes a supporting peripheral frame around the perimeter of the screen. Some screens are tensioned when they are installed in the screening machine and other screens are pre-tensioned in a frame prior to being installed in the machine. Typically associated with the screen deck are additional material handling elements that are moved with the screen and form walls or partitions above or below the screen for containing the liquid and/or particulate materials adjacent to the screen and directing them to appropriate outlets. These elements may comprise a top cover and a pan beneath the screen deck. In the case of screening machines with multiple screens or deck units, spacer pans or frames are provided between the multiple screens.
The screens are often removed from the screening machines for cleaning, replacement, readjustment, or installation of a screen of a different mesh size or the like. The screen is releasably mounted to a carrier, table or box to which vibratory motion is imparted, typically by one or more eccentric motors or other means of excitation. The carrier, table or box is referred to herein as a “vibratory carrier”. The vibratory carrier may be moved in oscillatory, vibratory, gyratory, gyratory reciprocating, fully gyratory, rotary or another type of motion or combinations thereof, all of which are herein collectively referred to as “vibratory” motion or variations of that term.
Screening machines of this type are used to separate or classify a wide variety of materials. Some of the materials which are processed in these screening machines are subject to various governmental regulations and/or other requirements during the handling and processing of the materials. These requirements and regulations often include cleanliness and sanitary standards for certification under the United States Department of Agriculture (USDA) regulations. USDA certification would then allow use of such a screening machine in the production and processing of protein products such as whey powder, milk powder, cheese, etc. The processing and handling of these and other types of products require USDA certification.
One challenge to satisfying the USDA certification standards for screen panels and screening machines has been a pre-tensioned screen panel that could meet the performance aspects of a standard screen panels as well as meet the cleanliness standards set by the USDA. Existing pre-tensioned screen panels typically have wire tensioned over a tubular frame and fastened to the frame using a Food and Drug Administration (FDA) approved epoxy. A bead of silicone may be located between the edge of the tubular frame and the wire interface to act as a damper to prevent wire fatigue. Various problems with such a screen panel design for satisfying the USDA requirements include the fact that the epoxy creates bubbles during the curing process and thereby create pores in which bacteria may collect and grow. Moreover, the epoxy interface with the screen panel components is often uncontrolled and does not have a clean appearance thereby creating more sites for potential mold growth. Additionally, when the epoxy presses through the screen wire, it creates fingers, or a series of protrusions that can either break off into the product being screened or create a cavity for bacteria to collect and grow.